In recent years, nonpolluting various coating materials containing organic solvents, such as paints or adhesives, is strongly demanded from the standpoints of preventing environmental pollution and ensuring safe and hygienic working atmospheres. For this reason, the use of emulsion type coating materials is extending and they are also attracting attention in the fields where solvent-based coating materials are used.
Under these circumstances, the emulsion type coating materials are also being required to have a high film performance. Although addition of colloidal silica to organic polymer emulsions is extensively conducted to improve the film performance, there is a drawback that due to the weak adhesion between the organic polymer and the silica particles, the durability of the resulting films, such as water resistance and alkali resistance, deteriorates unavoidably in a prolong period of time.
For the purpose of improving the adhesion between colloidal silica particles and organic polymers, i.e., the interfacial adhesion between each silica particle and the organic polymer, various proposals have been made. For example, JP-B-62-58630 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a surface-treating agent for metals which is obtained by incorporating a colloidal silica and a chromium compound into an acrylic resin obtained by the multi-stage emulsion polymerization of an organosilicon compound having both a reactive unsaturated group and an alkoxy group with several kinds of acrylic monomers. The agent can give a coating which imparts good corrosion resistance and coating suitability to the metal surface and, at the same time, is excellent in adhesion, deep draw processing properties, and anti-block properties. This surface-treating agent, however, has the following disadvantage. During the production of the acrylic resin, the alkoxy groups undergo hydrolysis to convert into silanol groups, resulting in, in some cases, formation of siloxane bonds (--SiOSi--) through subsequent condensation reactions. Since these portions which can exert an interaction with silica (i.e., alkoxy groups, silanol groups, and silaxane bonds formed by condensation of the silanol groups) are uniformly dispersed in the resulting acrylic polymer, the number of such portions present on the interface between the acrylic polymer and silica particles subsequently added is only small. Therefore, sufficient interfacial adhesion cannot be obtained.
JP-A-59-71316 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses an aqueous resin dispersion which is obtained by emulsion-copolymerizing an acrylic monomer, a styrene-type monomer, and an organosilicon compound having both a reactive unsaturated group and an alkoxy group in an aqueous medium in the presence of a colloidal silica. The dispersion forms a film having excellent durability and good resistance to stains. JP-A-61-155474 discloses an aqueous coating composition having functions such as durability, flame retardant properties, resistance to stains, dew condensation-preventive properties, and other properties, comprising an aqueous resin dispersion obtained by emulsion-polymerizing an acrylic monomer and an organosilicon compound having both a reactive unsaturated group and an alkoxy group in the presence of a colloidal silica, and a water-soluble or water-dispersible acrylic copolymer having an alkoxysilyl group as a binder component.
However, both of the above two proposals have the following drawback. Since in either case the emulsion copolymerization of an organosilicon compound with organic monomers is conducted in the presence of a colloidal silica, the radical copolymerization at olefin moieties, the condensation reaction of alkoxysilyl groups with Si-OH groups on the surface of colloidal silica particles, and the condensation reaction among alkoxysilyl groups proceed simultaneously. For this reason, it is difficult to complete the desired condensation reaction of alkoxysilyl groups with Si-OH groups on the surface of silica particles. Further, part of the colloidal silica particles do not take part in this condensation reaction. Therefore, the interfacial adhesion between the organic polymer and the colloidal silica particles cannot reach a sufficient level.
On the other hand, as one approach to the development of highly functional polymer materials, modification of organic polymers with silicones is being attempted in recent years so as to impart to the organic polymers the heat resistance, cold resistance, weathering resistance, flame retardant properties, and other excellent properties possessed by the silicones. However, since the silicone is an unusual polymer having a siloxane skeleton and, hence, has poor compatibility with ordinary polymers having carbon skeleton, compositions obtained by merely blending silicones with ordinary organic polymers cannot satisfactorily retain the desired properties of the silicones. It is, therefore, necessary to chemically bond a siloxane skeleton to a carbon skeleton by grafting or other means.
As such a technique, polymerization of a vinyl monomer in the presence of a polyorganosiloxane containing a vinyl or alkyl group is disclosed in, for example, JP-A-50-109282. This polymerization yields a graft copolymer, thus imparting improved impact strength to the resulting resin.
Furthermore, JP-A-60-252613, JP-A-61-106614, and JP-A-61-136510 propose to obtain a graft copolymer having a high degree of grafting and excellent impact strength by polymerizing a vinyl monomer in an emulsion of a polyorganosiloxane containing an acryloyl or methacryloyl group.
However, the graft copolymers obtained by the above methods are defective in that their mechanical properties such as impact resistance have not been improved sufficiently because polyorganosiloxanes only which are insufficient in strength are used as a rubber component. For the purpose of improving the insufficient strength of such polyorganosiloxanes, they are used in combination with tri- and tetrafunctional crosslinking agents such as an alkyltrialkoxysilane, tetraalkoxysilane, etc., but attainable improvements in strength are limited and a satisfactory polyorganosiloxane has not yet been obtained.
One possible approach to improve the above-described insufficient mechanical properties including impact resistance is to incorporate a reinforcing material such as silica into domains of a polyorganosiloxane which is one of the two major components of a graft copolymer. As a result of the incorporation of such a reinforcing material, not only the mechanical properties of the resulting graft copolymer are improved, but also new properties are expected to be imparted to the graft copolymer. In order to realize the above approach, a silicone emulsion in which a polyorganosiloxane and a silica particle are contained in the same micelle is required, and more preferably, an emulsion of colloidal silica-core silicone-shell particles each of which comprises a core which is a colloidal silica particle and a polyorganosiloxane shell covering the core through siloxane bonds is desired.
Although no literature has so far been found which reports or proposes a process clearly intended for the preparation of the above-described core-shell particles, there are some literature references disclosing processes which may result in formation of such core-shell particles. For example, JP-A-61-16929, JP-A-61-271352, and JP-A-61-272264 disclose a process for obtaining an aqueous emulsion of a silicone reinforced with colloidal silica particles, in which a hydroxyl-terminated polyorganosiloxane is condensed using a sulfonic acid-type emulsifying agent in the presence of an acidic colloidal silica. However, since the siloxane used as a raw material has a relatively high degree of polymerization, it is difficult for the initial homogenizing step to include the siloxane and a colloidal silica particle in the same micelle and, as a result, the above process necessarily yields an emulsion containing desired core-shell particles and also containing colloidal silica particles and polyorganosiloxane which have not participated in polycondensation. Therefore, even if this method is applied to the above-described process for producing a graft copolymer, it is difficult to exhibit the effect of the incorporation of colloidal silica.
JP-A-1-234468 discloses a reactive microgel composition which is obtained by mixing an organosilane compound having a polymerizable unsaturated double bond and an alkoxy group and, if necessary, other alkoxysilanes with a silica sol, and then subjecting the resulting mixture to hydrolysis and co-condensation, and the composition cures upon exposure to irradiation or heat to give films having excellent film properties. However, this composition cannot impart to organic polymers the heat resistance, cold resistance, weathering resistance, release properties, and other properties possessed by silicones, and is not intended for use in such a purpose. Further, the above reference does not suggest such use of the composition.